\begin{tikzpicture}[
    myarrow/.style={line width=3pt, -{Stealth[length=5mm]}, gray},
]
    \tikzset{
        oxygen/.pic={% 绘制 “氧原子”
            \shade[ball color=gray!20] (0, 0) circle (.5cm);
            \draw[pattern={mylines[angle=0, distance={3pt}]}] (0, 0) circle(.5cm);
        }
    }

    \begin{scope}[yshift=2cm]
        \shade[ball color=black!50] (0, 0) circle (.6cm);
        \draw (1.1, 0) pic {oxygen}
            node[below=0.6cm, xshift=-0.5cm] {氧化汞分子};

        \draw [myarrow] (3, 0) -- (4, 0);

        \shade[ball color=black!50] (5, 0) circle (.6cm) node [below=0.6cm] {汞原子};
        \node at (5.8, 0) {\Large $+$};
        \draw (6.6, 0) pic {oxygen} node[below=0.6cm] {氧原子};
    \end{scope}

    \begin{scope}
        \draw (0, 0) pic {oxygen} node[below=0.6cm] {氧原子};
        \node at (0.8, 0) {\Large $+$};
        \draw (1.6, 0) pic {oxygen} node[below=0.6cm] {氧原子};

        \draw [myarrow] (3, 0) -- (4, 0);

        \draw (5, 0) pic {oxygen}
              (6, 0) pic {oxygen}
              node[below=0.6cm, xshift=-0.5cm] {氧分子};
    \end{scope}
\end{tikzpicture}

